Hybrid integrated circuits used in automotive applications often employ surface-mounted integrated circuit packages. Such packages are typically electrically interconnected with their respective conductors on the substrate of the hybrid integrated circuit using wire bonding techniques, in which a very thin electrically-conductive wire, often on the order of up to about 0.3 millimeters in diameter, is bonded from the lead of the package and to a bond pad, which may be a thick film conductor or a bond pad which is soldered to the corresponding conductor. Bond pads serve to promote a durable and reliable bond between the wire, which is typically an aluminum alloy, and the conductor, which is typically formed from a different material than that of the wire, such as silver or its alloys and copper or its alloys.
In the past, suitable bonds between the leads of a package and the wire have been achieved with various bonding techniques, including soldering, thermosonic bonding, and ultrasonic bonding. Ultrasonic bonding techniques are advantageous in that fluxes and filler metals are not required, an electric current is not employed at the bond site, and melting of the members being joined does not occur. Instead, the lead and wire contact each other with a known force, and high frequency vibrations are transmitted through a bond tool to the wire, such that the wire and lead oscillate relative to each other in a plane parallel to their surfaces forming the bond interface. As such, only localized heating occurs between the wire and lead, and at levels which do not exceed the melting temperature of either the wire or lead. A bond joint is formed due to the shear stresses generated in the bond interface, which leads to elastic hysteresis, localized slip and plastic deformation of the contacting surfaces, such that surface films are disrupted and metal-to-metal contact is achieved.
While ultrasonic bonding techniques are used for the purpose of bonding thin aluminum wires to the leads of electronic packages, bonds having insufficient strength occur under mass production conditions. Resulting scrappage rates can be relatively high, depending on the particular application and the materials involved. To promote the compatibility of the aluminum wire and the package lead, which is often a pin or part of a leadframe formed from a nickel-base alloy, the package lead is often nickel plated.
For the purpose of enhancing the strength of the ultrasonic bond joint, conventional wisdom is to produce the nickel plating with a relatively rough microfinish. As used herein, a rough microfinish is generally characterized by grains and/or grain boundaries being visible under magnification, such that the plating surface has a dull or matte appearance. Yet, such techniques may still result in scrappage rates on the order of 25 percent or more in mass production, and may produce bonds which have significantly reduced reliability.
Thus, it would be desirable to enhance the integrity of ultrasonic bonds formed between the leads of an electronics package and its corresponding wires. A suitable solution would not significantly complicate the manufacturing or handling of the electronic package, yet be highly repeatable and reliable when used in mass production, particularly for the manufacture of electronic assemblies for the automotive industry.